\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\usetikzlibrary{positioning,shapes,arrows,snakes}
\definecolor{yellowish}{HTML}{FFE699}
\definecolor{grayish}{HTML}{D9D9D9} 
\tikzset{
   rect/.style={
      align=center,
      text=black,    
      minimum width=2.5cm,
      minimum height=2cm}
}
\tikzset{
   done/.style={rect, fill=grayish }
}
\tikzset{
   todo/.style={rect, fill=yellowish}
}
\tikzset{
   arrow/.style={line width=1mm}
}
\centering

\begin{tikzpicture}[font=\sffamily\large]
   \node [done] (broncodering) {Broncodering\\(compressie)};
   \node [todo, right=1.5cm of broncodering ]  (codering) {Kanaalcodering\\(foutcorrectie)};
   \node [done, right=1.5cm of codering ]    (modulatie) {Modulatie\\(bijv. PSK, QAM)};
   \node [done, right=1.5cm of modulatie	]  (dac) {DAC\\(digitaal-\\analoog-omzetter)};
   \node [done, right=1.5cm of dac	]  (tx) {RF Circuit\\(filters,verschuiving,\\versterking)};
   \node[rect, cloud, cloud puffs=15.7, cloud ignores aspect, minimum width=5cm, minimum height=2cm, align=center, fill=grayish, right=1.5cm of tx] (channel) {Draadloos\\kanaal};
   \node [done] at (0,-4)                   (rx) {RF Circuit\\(Filters,verschuiving,\\versterking)};
   \node [done, right=1.5cm of rx]      (adc) {ADC\\(analoog-\\digitaal-omzetter)};
   \node [done, right=1.5cm of adc]         (demod) {Demodulatie};
   \node [todo, right=1.5cm of demod]        (decodering) {Kanaal-\\decodering};
   \node [done, right=1.5cm of decodering]   (brondecodering) {Bron-\\decodering};
   \draw [arrow, stealth-](broncodering.west) -- ++(-2,0) node[left]{\emph{Data}};
   \draw[arrow, -stealth] (broncodering.east)--  (codering.west);
   \draw[arrow, -stealth] (codering.east)--  (modulatie.west);
   \draw[arrow, -stealth] (modulatie.east)-- (dac.west);
   \draw[arrow, -stealth] (dac.east)--     (tx.west);
   \draw[arrow, -stealth] (tx.east)--     (channel.west);
   \draw[arrow, -stealth] (channel.south) -- ++(0,-0.5) -| (rx.north);
   \draw[arrow, -stealth] (rx.east) --  (adc.west);
   \draw[arrow, -stealth] (adc.east)--    (demod.west);
   \draw[arrow, -stealth] (demod.east)--    (decodering.west);
   \draw[arrow, -stealth] (decodering.east)--      (brondecodering.west);
   \draw[arrow, -stealth] (brondecodering.east) --++(1,0) node[right]{\emph{Data (hopelijk)}};
\end{tikzpicture}
\end{document}